The present application concerns apparatus for sensing the magnitude of AC current flow and, more particularly, concerns novel current sensors utilizing split-conductor configurations and compensating for magnetic field and/or thermal effects.
Large magnitudes of alternating current flow are often measured by utilizing a current divider to split the current flow, whereby a reduced magnitude current is caused to flow in a shunting sensor branch. The reduced-magnitude current is sensed, through a current transformer, by an active load circuit. One such current-sensing transducer is described in U.S. Pat. No. 4,182,982 to J. E. Wolf et al., issued Jan. 8, 1980. This transducer 1 (shown in FIG. 1) utilizes a conductive member 2 having a main portion 2a through which flows a first current I.sub.1, which is a substantial portion of the total current I to be measured. A second current portion I.sub.2 flows through a second branch 2b of the transducer. It will be seen that, because the voltages across parallel portions 2a and 2b are equal, the ratio of portion resistances R.sub.1 and R.sub.2 should establish the ratio of currents I.sub.1 and I.sub.2 flowing in first and second portions 2a and 2b. However, current I.sub.1 causes a magnetic field B.sub.1 to be generated, which magnetic field induces an additional current I.sub.B in conductor portion 2b. While the current I.sub.2 flowing portion 2b also induces a current in portion 2a, as the ratio of currents I.sub.1 to I.sub.2 is generally large, e.g. greater than 10:1, the greatest effect is noted in the current I.sub.2 +I.sub.b flowing through conductor portion 2b; this current I.sub.2 +I.sub.b flows in the primary of a current transformer (not shown for purposes of simplicity) at the secondary winding of which is obtained an indication of the total current I flowing through transducer 1. The sensed load current is proportional to I.sub.B =KS(2.pi.f)(I.sub.1 -I.sub.2)/R.sub.2 .times.sin (2.pi.f-90.degree.), where S is the area of the loop formed about sensor opening 2c, f is the frequency of the AC current to be measured, and K is a geometry constant. It will be seen that the induced loop current I.sub.B is thus 90.degree. out-of-phase with the current I.sub.2 be measured. As both currents I.sub.2 and I.sub.B add vectorially, the actual measured current can have a significant magnitude and phase error between the total current I and the desired branch current I.sub.2.
Further, the prior art transducer 1 is prone to a self-heating effect which causes an increase in divider current I.sub.2 at higher currents, due to the positive temperature coefficient of the main-path resistance R.sub.1 and, therefore, increased power dissipation P.sub.1 =I.sub.1.sup.2 R.sub.1 in that resistance with increased current flow. That is, with the same voltage drop across portions 2a and 2b, the power dissipation in portion 2a will be greater than the power dissipation in portion 2b, and the relative power dissipations will cause the main-path resistance R.sub.1 to increase more than the resistance of shunt-path resistance R.sub.2, whereby I.sub.2 increases proportionally with the square of the total transducer current I.
It is desirable to provide current sensors which have increased accuracy by removal of the effects of induced current and/or self-heating phenomena.